This application claims priority to German patent application No. 01104338.7 filed Feb. 23, 2001.
The invention relates to a hydraulically supported steering system with a steering gear that is driven by a hydraulic piston/cylinder unit.
Hydraulically supported steering systems have been known for a long time. It is moreover well known that pronounced smooth running of the steering gear can positively effect a vehicle""s handling characteristics. However, smooth-running steering gears make the entire steering system susceptible to oscillations. Therefore, it is necessary to use steering dampers to effectively combat the oscillations in the steering system. An example of a hydraulically operated steering stabilizer is illustrated in U.S. Pat. No. 4,467,884.
Furthermore, undesired noise can develop in hydraulically supported steering systems (xe2x80x9cwater line knockingxe2x80x9d) when erratic pressure changes appear in the hydraulic system and pressure peaks reach the system""s return line. This can be caused for example, by external disturbances that act upon the steering wheel during driving. Hydrodynamic damping elements (xe2x80x9cthrottlesxe2x80x9d or xe2x80x9ccompensating volumesxe2x80x9d) are often used in hydraulically-supported steering systems for damping the undesired noise.
DE 28 38 151 A1, discloses a hydraulic piston/cylinder unit that can be used both as a steering system""s hydraulic support as well as a steering damper. Additionally, DE 40 29 156 A1 discloses that in order to suppress undesired oscillation or erratic pressure changes in a hydraulically supported steering system in which the piston/cylinder unit also acts as a steering damper, damping valves that are only operative in the supply direction to the cylinder subchambers can be advantageously installed into the hydraulic lines of the piston/cylinder unit""s cylinder subchambers.
It is furthermore known that to achieve optimum steering characteristics, it is advantageous for a hydraulically supported steering system to act as a function of speed. Large steering angle changes are often undertaken at low vehicle speeds (e.g., when maneuvering) in which a high internal damping in the hydraulic system is not desired High internal damping at low vehicle speeds in the hydraulic system thus limits the maximum flow of hydraulic fluid into the cylinder subchambers of the piston/cylinder unit, thereby limiting the unit""s regulating speed. If the driver undertakes faster regulating movements, this will cause the hydraulic steering assistance to fail. At higher vehicle speeds on the other hand, not only is hydraulic support of steering motions not required because of the much smaller control forces, it can even cause a drop in driving stability due to its susceptibility to oscillations. At higher vehicle speeds, complete shut-off of the hydraulic steering assistance, or at least a higher damping effect of the piston/cylinder unit operating as steering damper, is desired.
An example of a speed-dependent steering system is disclosed in DE 28 38 151 A1. This disclosure proposes a steering system comprising of a piston/cylinder unit that only operates as a steering aid at low vehicle speeds and only serves as a steering damper at higher speeds. For this, the vehicle speed is analyzed by electronic means and the piston/cylinder unit is only pressurized when necessary. However, this device as disclosed in the patent, has a number of drawbacks, the device firstly requires detection of the vehicle speed and electrical or electronic evaluation of this information, and secondly requires an electrically controlled power-assisted device for pneumatic control. Both factors increase the cost, and the steering system""s high complexity makes it susceptible to problems or failure, making it more difficult to use the system in the smaller sized cars.
EP 1 013 535 A1 discloses a hydraulically supported steering system, which works with variable throttle valves in the hydraulic supply lines to the cylinder subchambers, and whose flow resistance is determined from pressure difference between the pressure produced by the hydraulic pump and the pressure in the return line to the hydraulic fluid reservoir. The hydraulic pump is lock-synchronized with the combustion engine that drives the vehicle. Since the pressure generated by the hydraulic pump is a function of engine speed, the characteristic curve of the hydraulically supported steering system is a function of vehicle speed. However, the variable throttle valve that is required is mechanically expensive and requires extra hydraulic control lines, which make retrofitting difficult, particularly in existing old cars. Moreover, lock-synchronization of the combustion engine and hydraulic pump cannot be assumed in all cases, so the advantage of a steering system whose characteristic curve is a function of speed can""t be realized in all vehicles.
Finally, DE 196 51 500 C1, discloses a damping valve arrangement for a hydraulic steering system. A check valve and a throttle valve are combined within one component, whereby the throttle valve""s flow resistance adapts itself to the temperature-dependent viscosity of the hydraulic fluid. This is realized by using temperature-sensitive mechanical setting elements that vary the opening of a bypass path realized through throttle valve as a function of temperature. However, an adaptation of the throttle valve""s damping action to the respective driving situation cannot be inferred from the disclosure.
It is therefore an aspect of the present invention to have a hydraulically supported steering system whose piston/cylinder unit also acts as a steering damper. The damping characteristics of the unit automatically adapt themselves to the motor vehicle""s driving situation at hand. It is another aspect to design the throttle mechanisms in such a manner that they can be manufactured at low cost and are easy to integrate into existing hydraulically supported steering systems.
In accordance with the teachings of the preferred embodiment, a steering system comprises a piston/cylinder unit that supports the steering gear""s movement. The piston/cylinder unit comprises a piston, which is axially displaced within a working cylinder and divides the cylinder into two cylinder chambers. Each cylinder chamber is connected to a separate hydraulic line that serves both as pressure supply line and as a return line. Both hydraulic lines are also connected to a servo valve actuated by steering movements of the vehicle.
In another aspect of the invention, a self-regulating throttle mechanism, which features a variable flow resistance in at least one flow direction, is arranged in at least one hydraulic line. The flow of hydraulic fluid through the throttle mechanism determines the flow resistance of the self-regulating throttle mechanism in this flow direction. If there is a lower flow of hydraulic fluid through the throttle mechanism, then a higher flow resistance will arise and lead to an effective damping of oscillations and erratic pressure changes in the hydraulic system.
A lower flow of hydraulic fluid will occur, when the driver undertakes small driving angle corrections at higher vehicle speeds. There will consequently be less hydraulic support of the steering system at higher vehicle speeds, this will have a positive effect on the vehicle""s handling characteristics, especially on its driving stability. Moreover, disturbances on the vehicle""s wheels will result in motions of the steering gear, motions that the piston/cylinder unit acting as steering damper will dampen, especially when the steering column hasn""t been actuated. Without effective damping of the steering gear, these disturbances will transmit itself to the steering column in the form of a xe2x80x9cknockingxe2x80x9d that is uncomfortable to the driver. If the piston/cylinder unit acts as a steering damper, then only trivial compensating motions of the piston will occur within the working cylinder and they will be accompanied by only a small flow of hydraulic fluid into the cylinder chambers or out of the cylinder chambers, respectively.
When there is a high flow of hydraulic fluid through the self-regulating throttle mechanism, a small flow resistance will arise against it, so that an effective hydraulic support of the driver""s steering movements will occur. A high flow will occur, when the driver makes maneuvers at the lowest speed. In so doing, the driver executes the largest steering angle changes in the shortest time. If the internal damping within the hydraulic system that supports the steering movements is too strong, it will lead to a less adequate supply of hydraulic fluid into the cylinder chambers, such that the supporting force that the piston/cylinder unit exercises on the steering gear will decline significantly. In this case, the driver must use greater force in his/her steering movements, which causes negative effects on riding comfort. According to invention, the throttle mechanism is designed in such a manner that a lower flow resistance will occur through the throttle mechanism under the above described conditions. This will ensure that full hydraulic support will be available, even for strong steering movements.
An analogous mode of operating the throttle mechanism occurs when the throttle mechanism""s hydrodynamic damping with the characteristic curve described above adapts itself to the speed of the pressure change in the hydraulic line between servo valve and throttle mechanism or to the size of the dynamic pressure difference occurring in front of or behind the throttle mechanism, rather than reacting to the flow of hydraulic fluid through the throttle mechanism.
In another embodiment, the throttle mechanism only demonstrates one flow path. This greatly reduces the structural and production engineering cost of the throttle mechanism in comparison to the multipath damping valves known.
Special advantages result when the flow resistance through the throttle mechanism in flow direction basically changes continuously with the flow of hydraulic fluid by means of a suitable design of the throttle mechanism. The characteristics of the hydraulically supported steering system continuously adapt themselves to the driving requirements by means of the continuous change of the hydraulic system""s internal damping, especially of the piston/cylinder unit serving as steering damper, without the driver perceiving an abrupt change in the steering characteristics that he/she would sense as disturbing.
Other advantages result when the self-regulating throttle mechanism demonstrates an asymmetrical flow-resistance characteristic curve in relation to the hydraulic fluid""s flow direction, namely a xe2x80x9cblocked directionxe2x80x9d and a xe2x80x9cflow-through directionxe2x80x9d. It is particularly advantageous for this type of self-regulating throttle mechanisms to be arranged in both hydraulic lines. In respect to the pressure intake in the cylinder chambers, the throttle mechanisms are each advantageously arranged in the flow-through direction. Such a fitting arrangement makes it possible to quickly fill the particular cylinder chamber that is contributing to a force on the steering wheel in support of the steering motion, so that the full hydraulic supporting force is available, even for rapid steering motions. The high damping required for a steering damper occurs in the opposite flow direction.
Throttle mechanisms that demonstrate a high flow resistance in the blocked direction up to the hydraulic fluid""s maximum flow, in other words throttle mechanisms that realize high internal damping, can be used as an auxiliary comfort and safety feature. It is of particular advantage for the flow resistance in this blocked direction to remain essentially constant, independent of the flow of hydraulic fluid, until a threshold is reached. When the maximum flow is exceeded, however, the throttle mechanism xe2x80x9copensxe2x80x9d into a xe2x80x9csafety state;xe2x80x9d i.e., its flow resistance advantageously drops jump-like to a low value to realize the hydraulic system""s full supporting action.
In a particularly simple embodiment of the hydraulically supported steering system, the self-regulating throttle mechanism demonstrates, in the flow direction, a flow-through state with little flow resistance and a damping state with elevated flow resistance. These two states of the throttle mechanism already permit the elementary functions of the steering system according to invention to be realized, namely high internal damping at higher speeds and low internal damping for strong steering motions.
The embodiments that are explained in more detail in the examples also realize additional significant cost advantages for manufacturing and implementing the throttle mechanisms. They consist of few parts, inexpensive to manufacture, and can be designed in such a manner at low cost that they can be integrated into existing designs of hydraulically supported steering systems without the need to make major changes. In particular, this opens up the advantageous possibility of retrofitting older vehicles simply and inexpensively.
The above object and other objects, features, and advantages of the present invention will be readily appreciated by one of ordinary skill in the art in the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.